374. Guess Number Higher or Lower

题目：

We are playing the Guess Game. The game is as follows:

I pick a number from 1 to n. You have to guess which number I picked.

Every time you guess wrong, I'll tell you whether the number is higher or lower.

You call a pre-defined API guess(int num) which returns 3 possible results (-1, 1, or 0):

-1 : My number is lower
 1 : My number is higher
 0 : Congrats! You got it!

Example:

n = 10, I pick 6.

Return 6.

链接：

https://leetcode.com/problems/guess-number-higher-or-lower/#/description

3/17/2017

果然是简单题，但是要注意返回值的意义。

/* The guess API is defined in the parent class GuessGame.
   @param num, your guess
   @return -1 if my number is lower, 1 if my number is higher, otherwise return 0
      int guess(int num); */

public class Solution extends GuessGame {
    public int guessNumber(int n) {
        int lo = 1, hi = n;
        int mid;
        int ret;
        
        while (lo <= hi) {
            mid = lo + (hi - lo) / 2;
            ret = guess(mid);
            if (ret == 0) return mid;
            else if (ret > 0) {
                lo = mid + 1;
            } else {
                hi = mid - 1;
            }
        }
        return 0;
    }
}

官方解答里有更多内容，包括ternary search，以及与binary search的比较

https://leetcode.com/articles/guess-number-higher-or-lower/